In this application pyrolysis means converting fuel in inert conditions and in high temperature into gaseous state, which forms oil-like, different organic compounds comprising liquid as it condensed. Inert conditions mean oxygen-free conditions where no combustion of the fuel occurs. Tar burning is one example of long known prior art pyrolysis process.
One possibility for a pyrolysis process is a process based on using a fluidized bed technique. In this process solid feedstock is fed to a reactor comprising a fluidized bed of hot inert bed material e.g. sand. Inert fluidizing gas is used to fluidize the bed. The feedstock will mix with the bed material, pyrolyze and form pyrolysis gases that are conducted out from the reactor. The heat for the pyrolysis process can be provided for example arranging the pyrolysis reactor next to a fuel burning fluidized bed and circulating the thus heated bed material to the pyrolysis reactor. The solid feedstock used in pyrolysis is dried in a thermal dryer before it is fed to the pyrolysis reactor. Usually the dryer is a separate drying apparatus. Thus the biomass is first dried and then conducted to the pyrolysis reactor.
In the pyrolysis process fuel is pyrolyzed, formed gaseous compounds, i.e. pyrolysis gases are separated from the char residue and the pyrolysis gases are condensed into pyrolysis oil. Pyrolysis oil may be used as fuel or refined into different chemicals. Production of pyrolysis oil from different kinds of biomass, for example from wood fuels, is studied for replacing coal and heavy fuel oil.
Pyrolysis oil produced by pyrolysis contains optimally 25 weight-% water. If the water content is much higher, the product may end up to unwanted phase separation and end use of the pyrolysis oil becomes difficult or alcohol has to be added to the pyrolysis oil. On the other hand, if the water content is too low, the viscosity of the pyrolysis oil will increase and end usage will get more difficult. This means that the water content in pyrolysis oil has to be maintained in certain range. Water or moisture content varies with different biomass for example wood, wood chips, peat or other biomass. Reaching the 25 weight-% water or moisture content in pyrolysis oils means that the biomass has to be dried to moisture content 10 weight-% at most, preferably about 4 to 10 weight-% and more preferably 5 to 8 weight-%.
The moisture content of the dried biomass affects the water content of the produced pyrolysis oil. The problem with the prior art drying arrangements is that using separate drying apparatuses in drying of the biomass does not enable effective control of the water content of the produced pyrolysis oil. Thus it is difficult to adjust the pyrolysis process such that the water content produced pyrolysis oil is in optimal range. Also, as the desired moisture content of the biomass is as low as mentioned above, the biomass residence time in a dryer of a smaller size is very long. Another alternative is to increase the size of the dryer, which will result in increase of drying equipment investment price and space requirement.